Which of the following is NOT a cause of hypernatremia?

• A. Lack of water intake
• B. Excess water loss
• C. Central diabetes insipidus
• D. Hyperthyroidism

Answer: (D)

Hypernatremia happens when free water is lost or not replaced enough, so the sodium concentration in the blood rises. The body normally keeps sodium in balance with thirst and vasopressin (antidiuretic hormone) to concentrate urine. If water intake falls short or the kidneys dump a lot of dilute urine without enough water to compensate—as in central diabetes insipidus where vasopressin is deficient—the result is a rising serum sodium. Lack of water intake directly lowers total body water, and excessive water loss from sweating, fever, osmotic diuresis, or diarrhea can cause hypernatremia if not matched by drinking. Central diabetes insipidus is a classic cause because the inability to concentrate urine leads to substantial free-water loss.

Hyperthyroidism, however, does not directly cause hypernatremia. It can influence fluid balance in various ways, but it is not a typical driver of elevated serum sodium; if anything, thyroid disturbances are more commonly linked to hyponatremia in certain contexts due to shifts in water balance. Therefore, hyperthyroidism is not a cause of hypernatremia.